Well, instead of just saying it is not a big deal or it is let's look at why it isn't there as it might give us a insight to wither we can expect it to arrive in a future update.
Now, I've never used MMS so correct me if I am wrong but it can be used to send videos and pictures from one phone to the other. The videos, I imagine, are those crappy stamp sized .3gp ones that all phones can record and display. Well, looking at the iPhone specs page I see no mention of .3gp playback under video capabilities so iPhone can't playback those files anyway, so receiving them would be pointless.
The pictures you can send with MMS, if I remember correctly, are first shrunk into stamp sized ones before being sent. The iPhone has a pretty big high resolution screen and stamp sized pictures would look like crap on it anyway, so Apple though that it would be better to let people use email to get full resolution pictures. And since the iPhone has no file browser and no ability to save downloaded or received pictures or videos into its photo or video libraries then you would have to open the MMS every time you would like to look at you stamp sized picture, just like with mail.
These things coupled with the fact that few people use MMS anyway makes it easy to see why Apple decided to omit it. I mean, to implement MMS functions would have been easy as pie for Apple so them omitting such a easily implementable function suggests that they did it on purpose for a reason. Ergo, don't count on seeing MMS functionality in the future.
